1. Field of the Invention
The present invention relates generally to a shock absorber used for suspension systems of an automotive vehicle for absorbing vibration energy and more specifically to a shock absorber in which damping force can be adjusted automatically according to vehicle speed or manually according to the driver's preference.
2. Description of the Prior Art
When an automotive vehicle rattles over a rough road, the springs in the suspension systems for the vehicle compress and extend to absorb the vibration shock. However, since the springs continue to vibrate until they return to their original states, the spring vibration reduces the road-holding ability and riding comfort of the vehicle. A shock absorber is used for imposing a restraint on the spring vibration for insuring better road holding capability and better riding comfort. To obtain better riding comfort, a shock absorber generally provides greater damping action when extended than when compressed. This action is achieved by the use of valves which change the flow of fluid filled within the shock absorber. This shock absorber, which provides damping action during both extension and compression, is in common use on today's vehicles. Further, the faster the working speed of the piston of a shock absorber, the greater the damping force or damping action of the shock absorber. If the damping force is too great, too hard a ride is obtained; if too soft, too soft a ride is obtained.
In the conventional shock absorber, however, since the damping forces during both extension and compression are usually fixed when working speed of the piston of the shock absorber is uniform, the road-holding ability, the riding comfort, or the steering-wheel manipulability vary according to vehicle speed or road harshness.
To overcome the above-mentioned problems, there has been proposed a variable-damping-force hydraulic shock absorber provided with a motor, an adjuster, etc. in which part of working fluid within the upper and lower chambers is by-passed during operation through one of a plurality of orifices of various diameters selected by an orifice adjuster rotated with a motor. That is to say, since part of the amount of the working fluid through the upper and lower piston valves is diminished, a smaller damping force can be obtained than the conventional damping-force obtained by the working fluid through only the valves. In this case, the adjuster is controlled in response to a feedback signal detected by an angular position sensor attached thereto.
In such a prior-art variable-damping-force hydraulic shock absorber as described above, since the adjusting motor and the related reduction gear are mounted outside the shock absorber, there exist problems in that the adjuster rotating motor is subjected to damage due to rain water or mud adhering thereto during vehicle running, resulting in deterioration of motor durability (short lifetime) and, what is worse, a noisy sound is often produced while the damping force is being adjusted.
Further, in such a prior-art variable-damping-force hydraulic shock absorber as described above, a potentiometer is widely used as the angular position sensor in order to detect the position of the adjuster rotated by the motor. However, since the potentiometer is usually so constructed that a brush or a sliding contact slides on a resistance material formed on an insulating board in order to increase or decrease resistance, the adjuster is usually feedback-controlled in analog fashion, thus resulting in another problem in that it is relatively difficult to control the angular position of the adjuster accurately and quickly in response to feedback signals generated by the potentiometer.
Furthermore, in such a prior-art variable-damping-force hydraulic shock absorber as described above, since the construction thereof is such that fluid pressure within the fluid chamber is directly applied to the driven shaft of the motor or the reduction gear via the adjuster in the axial direction thereof, the driven shaft is subjected to an axial pressure, thus resulting in another problem in that a greater torque is required for the motor and therefore the size of the motor is large or the driven shaft is easily damaged when fluid pressure rises abnormally.